1. Field of the Invention
The present invention relates generally to integrated circuits and, more particularly, to an integrated circuit having a self repair circuit thereon.
2. Description of the Related Art
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present invention that are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Microprocessor-controlled integrated circuits are used in a wide variety of applications. Such applications include personal computers, vehicle control systems, telephone networks, and a host of consumer products. As is well known, microprocessors are essentially generic devices that perform specific functions under the control of a software program. This program is stored in a memory device coupled to the microprocessor. Not only does the microprocessor access a memory device to retrieve the program instructions, it also stores and retrieves data created during execution of the program in one or more memory devices.
There are a variety of different memory devices available for use in microprocessor-based systems. The type of memory device chosen for a specific function within a microprocessor-based system depends largely upon what features of the memory are best suited to perform the particular function. For instance, volatile memories, such as dynamic random access memories (DRAMs), must be continually powered in order to retain their contents, but they tend to provide greater storage capability and programming options and cycles than non-volatile memories, such as read only memories (ROMs). Non-volatile memories, on the other hand, traditionally permit only limited reprogramming, so they are often used to store operating programs and the like. For example, certain memories, such as electrically erasable programmable read only memories (EEPROMs) and Flash memories, permit limited reprogramming. However, such memories typically must be tied to a relatively high external voltage, e.g., 12 volts, and/or to a dedicated programming device to facilitate such reprogramming.
Contemporary semiconductor-based memory products use one or more arrays of memory cells, often referred to as a memory matrix. Such arrays are formed from a plurality of rows and columns of memory cells. Memory devices of this type often incorporated a high degree of redundancy in order to improve manufacturing yields. Present redundancy techniques in memory products include providing extra memory array columns and/or extra memory array rows that can be used to replace defective columns and/or rows. These redundant rows and columns are typically isolated from the memory matrix by antifuses. While a fuse is a low resistance line that passes current until the amperage on the line becomes high enough to xe2x80x9cblowxe2x80x9d the fuse to create an open circuit, an antifuse is normally an open circuit until the voltage across the antifuse becomes high enough to xe2x80x9cblowxe2x80x9d the antifuse to create a closed circuit. The normally open circuit created by the antifuse effectively isolates redundant rows and columns from the memory matrix unless they are needed. If a redundant row or column is needed, the antifuse isolating the selected redundant row or column is blown to create a closed circuit to add the redundant row or column to the memory matrix.
During initial testing of a memory circuit, which typically takes place before the memory circuit is even packaged, an external testing apparatus may exercise the memory circuit to determine whether all of the rows and columns of the memory matrix are functioning properly. If so, then the redundant rows and columns are not used. If not, however, the external testing apparatus may apply a high voltage to the antifuse of the selected redundant row or column to replace a nonfunctioning row or column and, thus, repair the memory matrix. The voltage used to blow the antifuse is typically above the power supply voltage (Vcc) of the memory circuit.
While external testing devices typically perform such testing and repair functions admirably, they are not without their drawbacks. First, the memory circuit to be tested must be coupled to the external testing apparatus, and the memory circuit must remain in the external testing apparatus until it is fully tested and fully repaired. This can be a time-consuming process even if only memory device is tested at a time. However, if the external testing apparatus is capable of testing multiple memory chips at a time, then all of the chips must remain in the testing apparatus until the last one is fully tested and fully repaired. Secondly, the external testing apparatus is somewhat limited in the speed at which it can conduct tests due to the long signal pathways that necessarily extend between the external testing apparatus and the integrated circuit under test.
In an effort to increase testing speed, integrated circuits may incorporate a built-in self test circuit. Built-in self test circuits can test multiple memory matrices at maximum clock speed. Indeed, built-in self test circuits can perform a full functionality test of a memory matrix and determine the most efficient repair. Typically, a built-in self test circuit sends a signal to an external testing apparatus so that the external testing apparatus can blow the antifuse or instruct a laser to blow an antifuse.
As is clear from the above discussion, although a built-in self test circuit can enhance the speed at which an integrated circuit is tested, it cannot perform the actual repair. Therefore, the one item that keeps the integrated circuit coupled to the external testing apparatus is the actual repair portion of the testing. Currently, the voltage and current required to blow an antifuse cannot be generated on the integrated circuit itself